Ethylene (C2H4) is a gaseous plant hormone that affects myriad developmental processes and fitness responses in plants, such as germination, flower and leaf senescence, fruit ripening, leaf abscission, root nodulation, programmed cell death, and responsiveness to stress and pathogen attack (Johnson, P. R. and Ecker J. R., Annu Rev Genet. 32, 227-254, 1998). Another effect of ethylene on plant growth is the so-called triple response of etiolated dicotyledoneous seedlings. This response is characterized by the inhibition of hypocotyl and root cell elongation, radial swelling of the hypocotyl, and exaggerated curvature of the apical hook. Over the past decade, genetic screens based on the triple response phenotype have identified more than a dozen genes involved in the ethylene response in plants. These genes can be divided into three distinct categories: constitutive triple response mutants (eto1, eto2 and eto3, ctr1 and ran1/ctr2); ethylene insensitive mutants (etr1, etr2, ein2, ein3, ein4, ein5, and ein6); and tissue-specific ethylene insensitive mutants (hls1, eir1, and several auxin resistant mutants).